1. Field of the Invention
This invention relates generally to analog-to-digital converters. More specifically, this application relates to reduced latency in an analog-to-digital converter. The disclosure is associated with high-speed pipeline analog-to-digital converters (ADC's), and may also be extended to other applications and ADC architectures. For example, the techniques described herein may be applied to algorithmic ADCs, also known as cyclic ADCs, to delta-sigma ADCs, to SAR ADCs, etc., and generally to any ADC that may require one or more digital-to-analog converters.
2. Description of the Related Art
Pipeline analog-to-digital converters (ADCs) are sub-ranging data converters in which a signal is quantized coarsely in several steps and the results of the different steps are then combined to achieve a high level of quantization accuracy. Pipeline ADCs are popular because they may operate at very high speeds (hundreds of MHz, depending on the technology employed) while achieving relatively large dynamic range.
Referring to FIG. 1A, a typical pipeline ADC 10 includes a plurality of stages 12, 14, . . . , L. In the first stage 12, the input Vin is converted using a flash ADC (not shown) and is combined with results from the subsequent stages 14, 16, . . . , L to form an output. As will be discussed in greater detail below, the error in each stage is determined by converting the output of that stage using a digital-to-analog converter. The difference between the input to the stage and the error is the “residue.” The residue for each stage is amplified and fed to the next stage and converted in the same fashion in the next stage. The output of the last stage L is provided to a backend ADC 16 which resolves the last bits. All outputs may be provided for time alignment and digital error correction 20.
As shown in FIG. 1B, a typical stage 100 in a pipeline ADC includes an input signal sampling network 108, an N-bit coarse ADC 102 with its own sampling network, an N-bit digital-to-analog converter 104 (DAC) and an amplifier 106. The sampling network 108 and flash ADC 102 sample the input signal 101 at the same time. The DAC 104 converts the quantized signal back into analog form and this signal is subtracted 110 from the main input signal 101. The residue 105 resulting from this operation is amplified using amplifier 106 in order to occupy, typically and in the absence of errors, a portion of the range of the following stage, for example, half. Ideally, the residue then consists only of quantization noise.
Three factors may limit the performance and speed of operation of a pipeline ADC: errors occurring within the DAC 104 known as element mismatches; errors occurring within the amplifier 106 due to gain and nonlinearity; and excessive delay through the flash ADC 102 and DAC 104 signal paths. All three result in degradation of the ADC linearity and signal-to-noise ratio (SNR).
FIG. 2 illustrates pipeline stage using a prior technique for addressing DAC 104 errors. The stage 200 includes an input signal sampling network 208, an N-bit coarse ADC 202 with its own sampling network, an M-bit digital-to-analog converter 104 (DAC) (where M>N) and an amplifier 206. Processing elements 232, 234 are added between the coarse ADC 202 and the DAC 204 and hence increase the delay in that path. This delay is critical in high speed operation, namely operation at rates of 250 MSPS and higher. An application of the technique of FIG. 2 has been proposed by others to address the problem of element mismatches in flash ADCs, by permuting reference thresholds presented to the comparators to improve the linearity of an ADC.
DAC and amplifier errors as described above are sometimes estimated and canceled or corrected using Dynamic Element Matching (DEM) and Harmonic Distortion Correction (HDC) techniques.
DEM takes thermometer-coded outputs of a coarse ADC and permutes them before they are provided to the connected DAC elements. The permutation matrix is such that every ADC output can reach every DAC input. The method of permutation sometimes randomizes the DAC errors, thereby creating a white spectrum, or shapes the errors such that the energy of an error signal occupies a region of frequencies outside the band of interest. In FIG. 2, the pipeline stage includes a DEM block 232 between the coarse ADC 202 and the DAC 204 and the proper placement relative to the stage digital outputs. The DEM block 232 can be implemented efficiently through the use of transmission gates. However, it introduces a finite delay which is non-negligible at high clock rates.
In a pipeline ADC, DEM is used with additional digital processing that estimates the DAC error signal and effectively removes it from the output. If this were not done, the DAC noise would decrease the SNR. This estimation and removal of the DAC errors is referred to in the literature as DAC noise cancellation (DNC).
Continuing with FIG. 2, a signal Σt is added 234 to the output of the coarse ADC 202. This signal consists of the sum of several random, independent sequences that are used in the estimation of the amplifier 206 errors, for example, gain and nonlinearity. The number of sequences depends on the order of nonlinearity that needs to be estimated: one sequence for linear gain error, three sequences for harmonic error, and so on.
Also shown in FIG. 2 is backend ADC 212, whose output is provided to amplifier 220 and HDC module 222, whose output is summed 230 with the output of DNC module 226.
In an HDC technique, the output of the residue amplifier 212 contains terms in the quantization noise of the coarse ADC 202, the random sequences and their interaction through the amplifier nonlinear characteristic. If the highest significant order of nonlinearity in the amplifier is 3, the output of the residue amplifier 212 contains one term proportional to a3 (Σt)3 where a3 is the third order nonlinearity coefficient and Σt=t1+t2+t3, three random sequences that can each take on values +A or −A where A is a constant quantity. Therefore, Σt is a four level signal that can take on values −3 A, −A, +A, +3 A. Since the product of random independent sequences is also a random and independent sequence, multiplying the (digitized) residue amplifier output by (t1, t2, t3) randomizes all terms except the one in a3 (Σt)3 which can be extracted with a lowpass filter.
A consequence of adding the random sequences to the output of the coarse ADC 202 is that the word length increases and the DAC 204 size and complexity increase accordingly. That is why the DAC 204 resolution M is greater than the coarse flash ADC 202 resolution N. In a typical implementation, M=N+3.
FIG. 3 is an example of a prior art implementation of a coarse ADC with N=2 bits resolution. This implementation is often called a flash ADC. Four comparators 302.n compare the input voltage on line 304 to four threshold voltages (THR1, THR2, THR3, THR4) respectively. In some implementations, the threshold voltages may be associated with a resistor ladder 306. Other voltage divider techniques may be used as well. If the input voltage is greater than THR1, then comparator 302.1 outputs a logical 1, otherwise it outputs a logical zero. Similarly, comparator 302.2 compares the input voltage with THR2 and so on. The output of the coarse ADC 206 is a digital word formed from the outputs of all the comparators. This word is often denominated a “thermometer code.” The number of logical ones contained in the thermometer code is the digital representation of the analog input voltage on line 304.
One example circuit embodying the comparator function is shown in FIG. 4. Only the circuitry to compare the input against THR1 (corresponding to 302.1) is shown for clarity of explanation. The circuit 302.1 includes eight switches 408, 410, 412, 414, 416, 418, 420, 422, two capacitors 402, 404, and a comparator 406. The switches are driven by periodic clocks denoted phase 1, phase 2. When phase 1 is true, phase 2 is false and vice versa. When phase 1 is true, the upper capacitor 402 is charged to the voltage THR1, whereas the lower capacitor 404 is charged to the input voltage. When phase 2 is true, the capacitors are connected to comparator 406. Many alternative circuit realizations of the comparator 302.1 exist in the relevant art, which accomplish the following function:
      Output    ⁢                  ⁢    1    =                    1                                                        if              ⁢                                                          ⁢                              (                                  input                  -                                      THR                    ⁢                                                                                  ⁢                    1                                                  )                                      >            0                    ,                                    0                    otherwise            
That is, the signal output 406 is a logical 1 if the input voltage is greater than the threshold voltage THR1, a logical zero otherwise.